In a gasoline engine, an air-fuel mixture is ignited, when a spark is generated in an ignition plug at an ignition timing, and combustion progresses, which is caused by propagation of a flame in a combustion chamber, to perform an expansion stroke. However, when the temperature inside of the combustion chamber partially becomes high in the expansion stroke, a spontaneous combustion phenomenon of fuel occurs before the combustion caused by propagation of the flame is performed. The phenomenon is called detonation (abnormal combustion), which abnormally and partially increases a temperature in the combustion chamber.
Especially, in a two-cycle engine in which one combustion cycle is performed during one rotation of a crankshaft, heat load of a piston or a cylinder is large, and thus temperature of an earth electrode part at the tip of an ignition plug or the like tends to partially become high, which causes detonation to occur easily. If a situation where detonation is occurring is allowed to stand, a part of metal constructing a cylinder is melt since a temperature of a part of a piston or the cylinder abnormally increases, which may lead the piston to be seized. Thus, if detonation occurs in the engine, it is necessary to detect the detonation promptly and take a step for inhibiting the occurrence of the detonation.
As shown in Japanese Patent Laid-Open Publication No. 6-108955 or Japanese Patent Laid-Open Publication No. 2002-357174, various control methods and control systems for inhibiting detonation occurred in two-cycle engines are proposed. In a conventional engine control system for inhibiting detonation, a sensor which detects occurrence of detonation is provided to perform one or both of an enrich control for enriching an air-fuel ratio of mixture and an ignition timing delay control for delaying an ignition timing of an engine, when the occurrence of detonation is detected by the sensor.
If the enrich control for enriching the air-fuel ratio of mixture or/and the ignition timing delay control for delaying the ignition timing of the engine is/are performed when detonation occurs in the engine, a temperature in a combustion chamber can be lowered, and thus detonation can be controlled.
In this type of the conventional control system, since the occurrence of detonation is controlled for the purpose of inhibiting a seizure of a piston, the occurrence of detonation to some extent is allowed. The reason the slight occurrence of detonation is allowed is that the seizure of the piston does not occur with the slight detonation and that shaft horse power of an engine is increased since combustion efficiency is improved in a state where the detonation occurs to some extent.
As described above, in the conventional engine control system which performs control of detonation, the occurrence of the detonation to some extent is allowed in order to try to improve an output of the engine. However, slight detonation may occur in a situation where an engine temperature is considerably high even if a driver closes a throttle valve to lower the output of the engine. If such a situation occurs, since the output of the engine is maintained at a high level even in a state where the throttle valve is closed, an adjustment for lowering the engine output to an output level desired by the driver cannot be performed by operating the throttle. If such a condition which is against the driver's intention suddenly occurs, under certain circumstances, there is a possibility that the driver cannot control the vehicle, which is undesirable.
In addition, if the detonation occurs in a condition where the throttle valve is closed, abnormal combustion sound (rattle sound) occurs, which may give discomfort to the driver and surrounding people.